In recent years, there is an increasing number of power consuming dwellings which generate an electric power by themselves with solar cells or fuel cells. However, there is a case where an amount of a power produced through the self-power generation exceeds an amount of a consumed electric power. In this case, a surplus power is discarded. Therefore, a technique for interchanging the surplus power between dwellings is suggested (e.g., Japanese patent application publication No. 2006-288162 (JP2006-288162A).